This paper investigates the amplitude and sweeping direction dependent behavior of nonlinear ultrasonic resonance spectroscopy for fatigue crack detection. The Contact Acoustic Nonlinearity (CAN) and the nonlinear resonance phenomena are illuminated via a reduced-order bilinear oscillator model. Unlike conventional linear ultrasonic spectroscopy, which would not change its pattern under different amplitudes of excitation or the frequency sweeping direction, the nonlinear resonance spectroscopy, on the other hand, may be noticeably influenced by both the wave amplitude and the loading history. Both up-tuning and down-tuning sweeping active sensing tests with various levels of excitation amplitudes are performed on a fatigued specimen. Short time Fourier transform is adopted to obtain the time-frequency features of the sensing signal. Corresponding to each excitation frequency, a nonlinear resonance index can be established based on the amplitude ratio between the superhamronic, the subharmonic, the mixed-frequency response components and the fundament frequency. The measured nonlinear resonance spectroscopy for a certain amplitude and frequency sweeping direction can be readily used to establish an instantaneous baseline. The spectroscopy of a different amplitude or frequency sweeping direction can be compared with such an instantaneous baseline and a Damage Index (DI) is obtained by measuring the deviation between the two spectra. Experimental investigations using an aluminum plate with rivet hole nucleated fatigue cracks are performed. A series of nonlinear spectroscopies are analyzed for both the pristine case and the damaged case. The spectral features for both cases are obtained to demonstrate the proposed fatigue crack detection methodology which may find its application for structural health monitoring (SHM). The paper finishes with summary, concluding remarks, and suggestions for future work.

The detailed characterization of a fluid flow following a convergent shock wave impinging a perturbed density interface is an extremely complex task as this flow combines geometry effects, compressibility effects and turbulence. Nonetheless, more understanding is necessary to be able to develop models that help accurately predict the flow behavior when occurring in engineering applications. Such an application is Inertial Confinement Fusion (ICF), where turbulent mixing induced by the interaction of the shock wave with the fuel pellet is detrimental to the fusion process. This interaction triggers mixing due to baroclinic vorticity deposition at the density interface in a phenomenon known as the Richtmyer-Meshkov Instability (RM). Next, the Rayleigh-Taylor Instability (RT) is driving the final growth of the mixing layer limited by secondary instabilities such as the Kelvin-Helmholtz Instability (KH). These classical hydrodynamic instabilities (HI) trigger the mixing process that leads ultimately to a highly-mixed fluid layer. For this study, we simulate a cylindrical Sulfur hexafluoride (SF 6 ) target immersed into an air medium. The incident shock wave is regarded as a Chisnell-type converging shock wave impinging into a perturbed cylindrical density discontinuity generated with a wave-like spatial perturbation spectra. Parameters of interest are the growth rate and width of the mixing layer at the density discontinuity. This study aims at describing and quantifying relevant aspects of these flows coupling mixing layer growth with perturbation modes.

Analysis of the electrical properties of a biological cell can provide useful information about its characteristic features, such as the intracellular composition, charge distribution and composition changes in cell membrane, as well as the extracellular environment. Electrical impedance spectroscopy of a cell suspension can be used to extract an average measure of the electrical properties of single cells. In sickle cell disease, the disease state of a sickle red blood cell is closely related to the intracellular hemoglobin composition and concentration. This study presents an electrical impedance measurement of sickle cell suspension with normal red blood cells as control. Electrical impedance spectra of cell suspensions are obtained in the range of 1000 Hz to 1MHz. Based on Maxwell’s mixture theory, average values of membrane capacitance and cytoplasm resistance of single cells are extracted for both normal and sickle blood samples. Comparing to traditional parallel-plate setup for cell suspension subjected to frequency sweep, this method requires low quantity of blood specimens and can be potentially valuable for patients that are already anemic.

Focal brain cooling has recently drawn attention as a less-invasive treatment for intractable epileptic patients. The objective of this study is to investigate the dependence of brain cooling rate on epileptic discharges (EDs) suppression with experiments using four epilepsy model rat. EDs were induced by Penicillin G in anesthetized rat, and cooled to 16 °C under four different time conditions (30, 60, 100 and 200 second, respectively). The ECoG obtained from the experiments were sorted into frequency band components which have physiological significance. The results of frequency analyses confirmed that the suppression of EDs have a cooling rate dependency, and power of four frequency bands (delta, theta, alpha and beta waves) in EDs are smaller when the cooling rate is slower. Our results suggested that slower cooling on brain surface can effectively suppress EDs and rapid brain cooling is not necessarily the best way. This finding implied that there are optimum cooling condition depending on the degree of EDs or epileptic symptoms.

Metamaterials demonstrate unique frequency dependent responses due to the presence of internal resonators; hence, it can be used to filter, absorb, cloak, or otherwise manipulate waves in unique ways. However, its applicability is normally limited to a very narrow frequency range (bandwidth) due to a dependency on linear resonance. The applications of these linear metamaterials are limited when used under the broadband excitation spectra that are common in real life applications. This paper numerically investigates the effect of introducing the two main classes of Duffing type cubic nonlinearities, namely monostable and bistable, on the attenuation bandwidth of an elasto-dynamic metamaterial. From the analysis, it is found that the attenuation bandwidth of a bistable nonlinear system is two to three times wider than that of an equivalent linear system; whereas, in case of a monostable system the bandwidth is remained same. In both cases, the attenuation bandwidth shifts towards the higher end of the frequency spectra and for higher nonlinearity and excitation amplitude, second transmission zone completely vanishes.

High frequency fluidic oscillators have been of scientific interest for many decades. Especially over the last couple of years fluidic oscillators became more important for active flow control applications. At the Institute of Jet Propulsion of the University of the German Federal Armed Forces Munich studies on different kinds of flow control methods were carried out on aerodynamically highly loaded low pressure turbine blades. On the basis of these studies, the most efficient way to trigger transition at low Reynolds numbers was found to be with fluidic oscillators at frequencies up to 10 kHz. Still, it is an open issue whether it is most efficient to trigger Tollmien-Schlichting waves, stimulate Kelvin-Helmholtz instabilities or simply induce a frequency independent disturbance in form of a periodic impulse for boundary layer control on aero-dynamically highly loaded low pressure turbine blades. To find an answer to these questions, a high frequency master-slave fluidic oscillator is introduced with an independent frequency and mass flow characteristic. Any frequency from the master oscillator’s characteristic can be chosen and the mass flow rate can be controlled with the slave oscillator. Contrary to concepts with fast switching valves or piezo actuators, this actuator is based on a working principle without the necessity of any moving and life limited parts. Based on experimental results, the characteristics of the master as well as the coupled oscillator are shown. The predictable operation of the coupled device is demonstrated in detail for a constant overall mass flow rate at discrete frequencies of 5 and 6 kHz. In addition, it is also shown that the mass flow can be varied with one master-slave arrangement by a factor of six while keeping the frequency constant at 5 or 6 kHz, respectively. Besides proof of concept these investigations focus on relevant parameters for active boundary layer and transition control. The frequency and velocity spectra of the coupled device are presented for constant frequency and constant mass flow operating points. Based on these results the improvement potential of the coupled oscillator for fundamental research on this topic is discussed.

For the present study, setting Strouhal Number as the control parameter, we perform numerical simulations for the flow over oscillating NACA-0012 airfoil at Reynolds Number 10 3 . Temporal profiles of the unsteady lift, and their respective spectra clearly indicate the solution to be a period-1 attractor for low Strouhal numbers. This study reveals that aerodynamic forces produced by the oscillating airfoils are independent of the initial kinematic conditions that proves existence of the limit cycle. Frequencies present in the oscillating lift force are composed of the fundamental harmonics, and its odd harmonics. Using these numerical simulations, we observe the shedding frequencies nearly equal to the excitation frequencies in all the cases. Hence, considering it as a primary resonance case, we model the unsteady lift force through a modified van der Pol oscillator. Using the method of multiple scales and spectral analysis of the steady-state CFD solutions, we estimate the frequencies and the damping terms in the reduced-order model. We prove the applicability of this model to all the planar motions of airfoil; heaving, pitching and flapping. With the increasing Strouhal number, the nonlinear damping terms for all types of motion approach similar magnitudes. Another important aspect in one of the currently-proposed model is capturing the time-averaged value of the aero-dynamic lift force.

In this paper, the 3-D unsteady turbulent flow inside a centrifugal pump is investigated by computational fluid dynamics (CFD) in ANSYS CFX, using Detached Eddy Simulation (DES) as the turbulence approach. The pump has a single end-suction and a single volute discharge. The impeller is semi-open (unshrouded with baseplate) and has five backswept blades and pump-out back blades. The CFD model of the pump consists of the inlet, the impeller, and the volute. A sliding mesh technique has been applied to the interfaces in order to allow unsteady interactions between the rotating impeller and the stationary parts. These unsteady interactions generate pressure fluctuations over the volute casing and blade surfaces that are hydroacoustic dipoles according to Lighthill’s acoustic analogy theory. The pressure fluctuation spectra at the volute tongue show that pressure fluctuations are generated mainly by the discrete components related to the impeller rotation at low frequencies, especially the blade-passing frequency (BPF) component. This component is approximately 1% of the reference dynamic pressure 0.5 ρν2 2 where ν 2 is the circumferential velocity at the impeller outlet. The discrete components with frequency larger than 4 times BPF are no longer obvious in the spectra. Compared to the experimental results, the CFD simulation predicts much lower amplitudes for the broad band pressure fluctuations. This is reasonable, because DES combines a classical Reynolds averaged Navier Stokes (RANS) simulation with elements of Large Eddy Simulation (LES), and both RANS and LES use average methods which filter out the high frequency fluctuations. Nevertheless, CFD is capable of accurately predict the BPF component. The pressure fluctuations on the casing and blade surfaces are extracted and modelled as the stationary and rotary dipoles, respectively, according to the Ffowcs Williams and Hawkings (FW-H) equation of the acoustic analogy theory. After Fast Fourier Transform, the spectra of the pressure fluctuations are obtained, and are used to predict the tonal hydrodynamic sound radiation at BPF and its low order harmonics. The sound radiation of casing surface dipoles is calculated by extracting the tonal components, and performing a surface integration with the fundamental solution to Helmholtz equation as the kernel. A frequency domain formulation of the FW-H equation with the moving surface dipole is employed to predict the tonal blade noise. The results from these acoustical simulations show that the sound power generated by the casing surface dipole is three orders of magnitude higher than that of the blade surface dipole, and the main hydroacoustic sources are located at the volute tongue.

Corrosion is the gradual deterioration of materials by chemical reaction with their environment, leading to the formation of less desirable material, which lacks in functionalities of the system. Corrosion resistant coatings have being an active subject of research since a while. Aiming at the replacement of toxic anti-corrosion pigments in organic coating industries, in the last two decades several environmentally friendly pigments have been studied, which involves passivating compounds such as molybdates, permanganates, vanadates, and tungstates. Cerium compounds are attaining more attention due to their potential as corrosion inhibitors. The present work aims at the experimental investigation on the effect of zirconium on corrosion resistance of ceria nanoparticles, in epoxy resins. Ceria and cerium zirconium mixed oxide nanoparticles was prepared by co-precipitation method. Ceria and cerium zirconium mixed oxide nanoparticles were dispersed in epoxy resin and the composites were spin coated on mild steel specimens. Surface studies was done using AFM and electrochemical studies were done one the coated surfaces using Zahnier IM6 modular electrochemical workstation. The Impedance spectra and surface structure studies showed highly improved corrosion resistance for ceria and cerium zirconium mixed oxide nanoparticles added epoxy primers as compared to unmodified epoxy resin primer. UV resistance of the coatings were found to be improved for ceria and cerium zirconium mixed oxide nanoparticles based coatings.

2D and steady analysis have been replaced by 3D and unsteady analysis because of dramatic improvements in computational environments. Analysis models that faithfully simulate actual products conventionally tend to be complicated and large scale. Therefore, the storage of analysis results has been increasing tremendously, and the dominant flow-field structure has been difficult to clarify. Data reduction by proper orthogonal decomposition (POD) is effective for simultaneously reducing the number of results of unsteady computational fluid dynamics (CFD) and for comprehending the dominant flow-field structure. However, only a few applications are used in the domain of industrial machinery. In this study, we applied the POD method to unsteady CFD results of a centrifugal blower in a vacuum cleaner and evaluated the benefits. We extracted a time series of static pressure distribution in the diffuser from unsteady CFD results corresponding to one rotation of the impeller, applied the POD to these data, and compared the results of an experiment. The results were that the first six modes had a 99.4% contribution in terms of the L 2 norm. In the scope of this research, the first six modes were revealed to surrogate the pressure fluctuation sufficiently. Also, the data storage was reduced to less than 2.0% of the original unsteady results. Next, frequency spectra were obtained by applying a discrete Fourier transform (DFT) to the expansion coefficients. The spectra of the expansion coefficients of the POD modes were found to have a peak near the blade passing frequencies (BPFs). The noise, the frequency of which is BPF, causes the majority of the noise that occurs in the diffuser. Therefore, we found by using both the POD and DFT that we could both reduce the dramatic data storage and extract the flow-field structures.

Rotating machines can be modeled at a basic level using lumped masses that are rotating about and attached using springs to an axis. Even such seemingly simple system can exhibit rich dynamics in the presence of time-varying terms in the governing differential equations. This paper investigates the dynamics of a rigid body with two attached rotors that rotate in the same plane. The system is parametrically-excited and the equations of motion are periodic in both rotor frequencies. The frequency spectra of the time responses show distinct side-band structures centered about the unforced natural frequencies. In addition to classical resonances, the stability diagrams generated using Floquet theory reveal instabilities at unexpected combinations of the forcing and natural frequencies. The harmonic balance method is employed to verify the stability boundaries obtained using Floquet theory. The study reveals safe regimes of parameter combinations that can help prevent the onset of instability in such systems.

The intermittent contact dynamics of a scanning probe microscopy (SPM) micro-cantilever are investigated in the context of high speed imaging in contact mode. At high scan speeds the cantilever can completely detach from the sample surface, and this lowers the achievable image resolution and limits the imaging speed. An analysis is performed, modeling the micro-cantilever as an Euler-Bernoulli beam and approximating the effect of the tip’s contact with the surface by an attached spring with an end mass that is subjected to attractive/repulsive interaction force. At low scan speeds, the cantilever follows the surface profile, while the frequency spectra exhibit a number of side-bands, while at higher speeds, the contact is intermittent. The sensitivity of the cantilever’s deflection varies along the length and hence the image resolution strongly depends on the point selected for optical laser deflection.

Fast Fourier Transform (FFT) calculates integer harmonic components, but ignores other frequency components such as non-integer harmonics and non-harmonics. In a practical sampling problem, the length of a sampled signal is finite and the sampling time is minimized. Therefore, it is necessary for interpolation to improve the accuracy of frequency analysis considering 1/m harmonic components and zero-padding (ZP) technique. The proposed method was applied to a 3D RF profile measurement system and the resolution was improved to 1/m.

Potential analogs between dynamics induced by periodic passage through a bifurcation critical value and the nonlinear dynamics associated with the aerodynamic dynamic stall problem are presented for the first time. Koopman operator methods are used to study the spectral features of a streamwise oscillating cylinder which exhibits wake dynamics due to externally forced oscillations through a Hopf bifurcation critical value. Koopman decomposition results show that the system transitions to a more continuous spectrum compared to the discrete spectrum associated with a stationary cylinder in post-critical flow. Finally, Fourier analysis of flow variables associated with an oscillating airfoil under dynamic stall conditions were compared with the oscillating cylinder spectra. The spectral characteristics of the two systems exhibited similar frequency broadening behavior induced by the externally forced oscillations. Therefore, the results indicate that the nonlinear dynamics associated with dynamic stall appear to have strong linkages to a system oscillating through a bifurcation critical value.

Recently Additive Manufacturing (AM) has been hailed as the “third industrial revolution” by The Economist magazine [April-2012]. Precision of the product manufactured by AM largely depends on the on line process diagnostics and control. AM caters to the quest for a material to suit the service performance, which is almost as old as the human civilization. An enabling technology which can build, repair or reconfigure components layer by layer or even pixel by pixel with appropriate materials to match the performance will enhance the productivity and thus reduce energy consumption. With the globalization, “Economic Space” for an organization is now spreads all across the globe. The promise of AM for Global Platform for precision additive manufacturing largely depends on the speed and accuracy of in-situ optical diagnostics and its capability to integrate with the process control. The two main groups of AM are powder bed (e.g. Laser Sintering) and pneumatically delivered powder (e.g. Direct Metal Deposition [DMD]) to fabricate components. DMD has closed loop capability, which enables better dimension and thermal cycle control. This enables one to deposit different material at different pixels with a given height directly from a CAD drawing. The feed back loop also controls the thermal cycle. New optical Sensors are either developed or being developed to control geometry using imaging, cooling rate by monitoring temperature, microstructure, temperature and composition using optical spectra. Ultimately these sensors will enable one to “Certify as you Build”. Flexibility of the process is enormous and essentially it is an enabling technology to materialize many a design. Several cases will be discussed to demonstrate the additional capabilities possible with the new sensors. Conceptually one can seat in Singapore and fabricate in Shanghai. Such systems will be a natural choice for a Global “Economic Space”.

By minimizing the differences between measured and predicted transmissivity spectra, an inverse radiation model was developed to reconstruct temperature and species concentration from homogeneous gas media. The model was validated by retrieving temperatures and concentrations from experimental medium-resolution CO 2 transmissivity data obtained by Bharadwaj and Modest [1, 2]. Optimal wavenumber ranges for CO 2 transmissivity measured across a wide range of temperatures and concentrations were determined according to the performance of inverse calculations. Results indicate that the inverse radiation model works well and the inverse radiation technique is superior to thermocouples and flow meters for measurements of temperature and gas concentration.

Water percolation into coating-metal interface is usually the main cause for the deterioration of corrosion protective property of organic coatings, which leads to coating delamination and under film corrosion. Recently, flowing fluid has received attention due to its capability to accelerate the degradation of materials. A plethora of works have focused on the corrosion of metallic materials accelerated by the flow of working fluids, while few studies have investigated the flow accelerated degrading behavior of organic coatings. For organic coatings, flowing fluid above the coating surface affects corrosion by enhancing the water percolation and by abrading the surface due to wall shear stress. Hence, it is of great importance to understand the influence of flowing fluids on the degradation of corrosion protective organic coatings. In this study, a commercially available epoxy based clear coating and pigmented marine coating were exposed to the laminar flow as well as stationary immersion. The laminar flow was pressure driven and confined in a newly designed flow channel. A 3.5 wt% sodium chloride solution was employed as the working fluid with a variety of flow rates. The corrosion protective properties of organic coatings were monitored inline by Electrochemical Impedance Spectroscopy (EIS) measurement. Equivalent circuit models were employed to interpret the EIS spectra. The time evolution of coating resistance and capacitance obtained from the model was studied to demonstrate the coating degradation. Thickness, gloss, and other topography characterizations were conducted to facilitate the assessment of the corrosion. The immersing solutions were measured by pH and conductivity meters as well as Fourier Transform Infrared Spectrometer (FTIR) to trace coating degradation products as they leached out from the coating. Initial attempts to acquire acceleration factors and predict service lifetime of organic coatings were also conducted.

Unbalanced response of cracked rotor-bearing system under time-dependent base movements is studied in this paper. Three base angular motions, including the rolling, pitching and yawing motions, are assumed to be sinusoidal perturbations superimposed upon constant terms. Both the open and breathing transverse cracks are considered in the analysis. The finite element model is established for the base excited rotor-bearing system with open or breathing cracks. Considering the time-varying base movements and transverse cracks, the second order differential equations of the system will not only have time-periodic gyroscopic and stiffness coefficients, but also the multi-frequency external excitations. An improved harmonic balance method is introduced to obtain the steady-state response of the system under both base and unbalance excitations. The whirling frequencies of the equivalent time-invariant system, orbits of shaft center, response spectra and frequency response characteristics, are analyzed accordingly. The effects of various base angular motions, frequency and amplitude of base excitations, and crack depths on the system dynamic behaviors are considered in the discussions.

In this paper, we consider a composite embedded in a homogenous medium and present a method to design the composite’s microstructure so that when it is subjected to an incident acoustic wave, it does not reflect the incoming wave and also dissipates most of it. High dissipation is achieved through a metamaterial design which is naturally characterized by internal resonance. The effective impedance of the metamaterial is evaluated through dynamic homogenization technique and it is used to design a quarter wave transformer to enforce zero reflection at the interface between the metamaterial composite and its surrounding medium. Transfer matrix method is used to calculate the reflection/transmission spectra at the above mentioned interface. Results show that the reflection at the interface is close to zero at the designed frequency and a large fraction of the wave energy which is transmitted across the interface is attenuated.

An investigation is performed on the scalability of broadband noise sources from separated flows in internal pipe systems. Broadband sources from for example wellhead chokes, bends and valves can potentially excite subsea manifolds through fluid acoustic coupling and fluid structural coupling. The focus of the current work is evaluation and improvement of scaling laws for collapse of sound power spectra. The approach proposed here is to use steady-state Computational Fluid Dynamics [CFD] to better estimate the properties of the flow in order to improve the scaling law and obtain a universal broadband spectrum. Steady Reynolds Averaged Navier-Stokes [RANS] simulations of several bend and orifice geometries have been performed. A surface acoustic power model based on modeled turbulent quantities is implemented. Based on the RANS data, more advanced models for scaling have been developed. Experimental sound power spectra from literature of the simulated geometries are scaled using different methodologies in both amplitude and frequency. When a new scaling based on CFD modeled surface acoustic power was used, a universal collapse among geometries occurred. Using CFD, the velocity in the high-speed sound-producing region is obtained, as well as a more accurate length scaling in order to improve the frequency scaling. A vast improvement in collapse over different geometries is achieved. The current work indicates that a universal collapse might indeed be present. The methodology does not require high fidelity calculations and is thus easy to implement. By comparing original and new scaling laws, it turns out that the ratio of fluctuating drag over steady drag can vary among geometries.